Abstract: Soil water scarcity, nutrient impoverishment, and low hydro-thermal-nutrient use efficiency are critical constraints limiting agricultural sustainability in the dryland loess regions of Northern China. This study aimed to evaluate the adaptability and underlying mechanisms of synergistic soil improvement through the combined application of flue gas desulfurization (FGD) gypsum and glutamic acid fermentation by-products (residue and liquid). A systematic field plot experiment was conducted in the Jinzhong dryland region, Shanxi Province, during an extremely arid growing season in 2024. The experiment comprised seven treatments: a control (CK, without amendment), and two composite amendments applied at three rates (7.5, 15.0, and 22.5 t ha⁻¹). Amendment-I was formulated with 60% FGDG and 40% glutamic acid residue, while Amendment-II consisted of 60% FGD gypsum and 40% glutamic acid liquid. Key soil parameters in the 0–20 cm layer, water-soluble ion compositions, and maize agronomic traits were determined at the harvest stage. The results demonstrated that both composite amendments exerted significant modulatory effects on soil water-salt-nutrient dynamics. In terms of hydrological properties, Amendment-II exhibited a highly stable water-retention effect（P<0.05）, increasing soil water content by 22.9% to 30.3% across all dosages, whereas Amendment-I showed a significant effect (25.6% increase, P<0.05) only at the highest rate (22.5 t ha-1). Regarding soil reaction, both amendments significantly mitigated alkalinity（P<0.05）, reducing pH values by 0.3 to 0.7 units. Specifically, Amendment-I demonstrated a superior acidification potential (0.5–0.7 unit reduction) due to its lower raw pH (2.51) and higher residual mineral acidity. Concurrently, soil electrical conductivity (EC) significantly increased from a baseline of 134 μS cm⁻¹ to ranges of 235.0%–461.0% (Amendment-I) and 115.0%–320.0% (Amendment-II) relative to the CK. This EC enhancement was primarily driven by the massive input of beneficial water-soluble ions, with Ca²⁺ concentrations increasing by 314.3%–757.1% (Amendment-I) and SO₄^2- concentrations reaching 18.5 to 56.0 times the levels found in the CK. Nutrient availability was significantly enhanced through the synergistic mechanisms of inorganic structural improvement and organic fertility supplementation. Amendment-II showed more pronounced effects on soil organic carbon (SOC) and alkali-hydrolyzable nitrogen (AN), with increments of 47.8%–56.0% and 17.4%–26.7%, respectively. Conversely, Amendment-I was more effective in activating available phosphorus (AP), exhibiting dramatic increases of 113.2%–252.0%, likely due to the localized dissolution of mineral phosphates and the chelation effect of organic acids in the fermentation residue. Maize grain yield exhibited a distinct dose-dependent response to amendment application. Amendment-I achieved a maximum yield increment of 47.0%, significantly outperforming the 30.1% increase recorded for Amendment-II. Random Forest modeling identified soil EC, AN, SO₄^2-, Ca²⁺, SOC, AP, AK, and Mg²⁺ as the most critical factors influencing yield, with EC contributing the highest relative importance (14.6%). This confirms that under the nutrient-limited conditions of dryland loess, the input of medium-element cations (Ca²⁺ and Mg²⁺) and anions (SO₄^2-) primarily serves a nutritional and stress-resilience function rather than inducing salinity stress. Agronomically, yield improvements were largely attributed to enhanced hundred-grain weight (up to 24.5% increase) and ear diameter, reflecting optimized source-sink dynamics during the drought-affected grain filling stage.In conclusion, the synergetic application of Amendment-I at a rate of 22.5 t ha⁻¹ is recommended as the optimal strategy for soil reclamation and yield enhancement in the Jinzhong dryland region. While these findings demonstrate substantial short-term benefits under extreme drought conditions, multi-year fixed-site monitoring is essential to evaluate the long-term environmental safety, salt accumulation kinetics, and the potential bioaccumulation of trace heavy metals. This study provides a scientific basis for the high-value utilization of industrial by-products in dryland agriculture, balancing the goals of soil reclamation and crop productivity